John Deere lawn tractor engine

My L120 lawn tractor broke the ring gear that is under the flywheel the starter engages with to start the engine. The one on the engine is held in place by three rivets. The one in the parts book shows it is held in place by four screws and nuts. The local dealer said to replace the whole flywheel instead of just the gear.
Was this a parts change and no original parts are made for the engine?
The engine has this on it.
Model 406777 Type 0139E1 Code 040102YC
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Dealer gave you good advice. If you wish just drill and tap the old flywheel so you can bolt on the new ring gear.
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